The present invention relates to a method for fabricating a semiconductor device and a semiconductor device, and particularly relates to a method for fabricating a semiconductor device including an oxide film which is formed by solution oxidation and, furthermore, into which nitrogen is introduced and the semiconductor device.
Among a plurality of transistors formed on a semiconductor substrate, for example, a transistor in a CMOS (complementary metal oxide semiconductor) device has a gate insulating film having a more and more reduced thickness for the purpose of improving the driving ability of a semiconductor device. In recent years, a gate insulating film having a thickness of 1–3 nm is required for such a transistor. On the other hand, in another transistor which is not required to perform a high speed operation but is required to use a relatively high voltage such as an input/output signal, a gate insulating film has to have a relatively great thickness in order to suppress a leakage current in the gate insulating film. In this case, a desired thickness of the gate insulating film is 7–10 nm. Since these transistors described above are formed in the semiconductor device, two or more different gate insulating films having different thicknesses have to be formed on the same substrate.
Conventionally, thermal oxidation (e.g., see S. M. Sze, VLSI technology, McGraw-Hill, 1984, pp.131–168) which allows formation of an oxide film with excellent properties as a gate insulating film has been mainly used in oxidizing a semiconductor device to form a gate insulating film. To form two different gate insulating films having different thicknesses on a semiconductor substrate, a method is used in which after a first gate insulating film has been formed by thermal oxidation, part of the first gate insulating film is removed by patterning and then a second gate insulating film is formed by thermal oxidation in a region of the semiconductor substrate from which the first gate insulating film has been removed. Moreover, besides thermal oxidation, use of various other methods for forming a gate insulating film has been examined (e.g., see Japanese Patent Publication No. 2937817, Japanese Unexamined Patent Publication No. 10-50701, Japanese Unexamined Patent Publication No. 10-223629, Japanese Unexamined Patent Publication No. 11-214386, and Japanese Unexamined Patent Publication No. 2002-64093).
As a technique for reducing the thickness of a gate insulating film for the purpose of improvement of the driving ability of a semiconductor device, a method in which nitrogen is introduced into a gate insulating film by annealing in nitrogen monoxide so as to reduce an electrical film thickness has been used. An electrical film thickness is a thickness measured in terms of electrostatic capacity. Even with the same physical thickness, the larger dielectric constant a film has, the smaller the thickness of the film is indicated. Oxynitride into which nitrogen is introduced has a larger dielectric constant than that of silicon dioxide. Therefore, by introducing nitrogen, an electrical film thickness is reduced, so that the driving ability of a transistor is improved. As a method for introducing nitrogen into a gate insulating film, i.e., a silicon dioxide film, a method using a plasma is known (e.g., Japanese Unexamined Patent Publication No. 10-79509).
Moreover, when nitrogen is introduced into a gate insulating film, as described in Japanese Unexamined Patent No. 10-79509, a dopant with which a gate electrode has been doped is prevented from reaching a substrate through the gate insulating film. This will be described further in detail.
In a CMOS transistor, a dual gate structure in which boron is introduced as a dopant into a gate electrode of a p-channel transistor and phosphorous is introduced as a dopant into a gate electrode of an n-channel transistor is used. Boron has a larger diffusion constant than that of phosphorous and thus is diffused in a gate insulating film through thermal treatment performed after the transistor has been formed. Thus, boron easily reaches a channel region. This phenomenon is called boron leakage and causes a large change in a threshold voltage, reduction in the driving ability of a transistor and the like. Especially, the smaller thickness a gate insulating film has, the larger the boron leakage becomes. However, if nitrogen is introduced into a gate insulating film, the boron leakage can be suppressed.
Problems that the Invention is to Solve
In a method for forming a plurality of gate insulating films according to the known method, a first gate insulating film is etched through wafer cleaning performed after a photoresist has been removed, so that the thickness of the first gate insulating film is once reduced. Then, when a second gate oxide film is formed, the thickness of the first insulating film is increased this time. This causes reduction in controllability of the thickness of the first gate insulating film, and also, in terms of film quality, it is very difficult to control the film quality of the first insulating film which has undergone through etching and additional oxidation.
Furthermore, assume that three different gate insulating films having different thicknesses (e.g., 7 nm, 3 nm and 1.5 nm) are formed. A second gate insulating film has a relatively small thickness, i.e., 3 nm. Thus, the second gate insulating film is more largely influenced by reduction and increase in a film thickness caused in forming a third gate insulating film having a thickness of 1.5 nm than a first gate insulating film having a thickness of 7 nm. That is to say, it is very difficult to control the thickness of the second gate insulating film so that the second gate insulating film has a constant thickness at any time. Accordingly, the ratio of an additional portion formed through additional oxidation to the entire thickness of the second gate insulating film is increased. Therefore, the quality of the entire second gate insulating film is largely reduced.
Moreover, assume that the thickness of a gate insulating film is reduced. When an oxynitride film is obtained as a gate insulating film according to a method described in Japanese Unexamined Patent Publication No. 10-79509, the electron energy of a nitrogen plasma is very high, i.e., about 50–1000 eV and this becomes a problem. For example, assume that a gate insulating film into which nitrogen is to be introduced has a thickness of 1.5 nm. Even when a nitrogen plasma has an energy of the lower limit of the energy range described in Japanese Unexamined Patent Publication No. 10-79509, i.e., 50 eV, the nitrogen plasma easily goes through the gate insulating film to nitride a silicon substrate as well. As a result, even though the thickness of the gate insulating film was about 1.5 nm before an exposure to the nitrogen plasma, the silicon substrate has been nitrided after the exposure to the nitrogen plasma, so that the total thickness of part of the gate insulating film which has been nitrided is over 2 nm. Thus, even though an oxide film having a thickness of 1.5 nm is formed, an oxynitride film having a small thickness can not be obtained. As a matter of course, the known method can not be used with a gate insulating film having a thickness of about 1 nm. Furthermore, when a nitrogen plasma has a higher energy than 50 eV, use of the known method is out of question. Nnitriding of a silicon substrate causes not only increase in a film thickness but also reduction in a driving force resulting from reduction in mobility or reduction in reliability.
The present invention has been devised in view of the above-described problems. It is therefore an object of the present invention is to provide a method for a fabricating a semiconductor device in which a gate insulating film having a small thickness allowing a high-speed operation can be formed with excellent film thickness control and furthermore, nitriding can be performed so as not to reach a semiconductor substrate, and a semiconductor device which includes a gate insulating film having a small thickness and excellent quality and in which a semiconductor substrate is hardly nitrided.